Field of Invention
This invention relates to a method for manufacturing a vessel for storing radioactive waste, and more particularly, to a method for manufacturing a vessel which withstands the heat generated by the radioactive waste without deforming utilizing a conventional vessel.
At nuclear power stations, the nuclear fuel which is used as an energy source is stored in a specific vessel after using so as to carry it to a storehouse for storing radioactive waste. The specific vessel is made of, for example, a non-corrosive stainless steel and the shape of such vessels differs depending upon the manufacturer. The conventional vessel for storing radioactive waste is substantially a triple walled vessel, in which lead (Pb) fills a center space formed between an innermost space and an outermost space in order to shield the environment which is exterior the vessel from radioactive waste contained in the innermost space.
A method for filling lead between the two spaces comprises filling the center space with molten lead and gradually solidifying the molten lead from the bottom of the vessel. At this time, unless the speed of the solidification of the lead is carefully controlled, countless air gaps form between the lead at the surface of the container wall and the walls of vessel containing the lead. If such air gaps are distributed over a wide area, the heat produced by the radioactive waste in the vessel cannot be dissipated effectively so that the temperature of the cooling water in the proximity of the gaps increases. The vessel may then distort or deform due to excessive heat forming in a localized area. Such deformation of the vessel may result in the leakage of radioactivity.
The formation of air gaps depends on the extent of the adhesion of the lead to the stainless steel walls of the vessel during the solidification of the lead. However, when adding molten lead to the vessel's center space i.e., between the innermost space and the outermost space, the desired adhesion of the lead to the stainless steel wall does not occur since an alloy of stainless steel and lead cannot be made.
In the prior art, in order to improve the extent of the adhesion between lead and stainless steel during the solidification of lead, a mixture of tin and lead (Sn-Pb) is plated on the surface of the stainless steel as a bonding material. Thus, after the surface of the stainless steel is cleaned, for example by iron particle blasting or by pickling with chemicals, zinc chloride is deposited on the surface of the stainless steel vessel as a flux and the surface thereof is then heated with a torch, and a tin-lead mixture is plated on the surface of the stainless steel vessel.
However, in the method described above, the melting point of zinc is 273.degree. C., which is very low. Especially, the melting point of zinc deposited on the surface of the stainless steel vessel becomes lower since an alloy of zinc and lead is made when heating the surface of the stainless steel with the torch. For example, as illustrated in FIG. 2, the melting point of ZnPb is 190.degree. C. Therefore, at the process of preheating the stainless steel vessel, for example, to approximately 300.degree. C., just before filling lead, the Tin/Pb melts from the vessel wall. Hence, the tin-lead mixture does not function very well as a bonding material when filling the vessel with lead.
It is an object of the present invention to provide a method for manufacturing a vessel for storing radioactive waste.
It is another object of the present invention to provide a method for filling a conventional triple walled stainless steel vessel for storing radioactive waste with lead, in which the center space stainless steel walls which contain the lead therebetween are plated with copper to enable adhesion of the lead to the surface of the stainless steel under processing temperature to thereby inhibit the formation of countless air gaps between the interface of the stainless steel walls and the lead within the center space.